BEHAVE WG C. Jennings
Internet-Draft Cisco Systems
Expires: August 14, 2005 February 13, 2005
NAT Classification Test Results
draft-jennings-behave-test-results-00
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Copyright (C) The Internet Society (2005).
Abstract
IETF has several groups that are considering the impact of NATs on
various protocols. Having a classification of the types of NATs that
are being developed and deployed is useful in gauging the impact of
various solutions. This draft records the results of classifying
NATs.
This draft is not complete and has only a few test results but it is
worth discussing all the testing we wish to do before all the test
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results are collected.
This work is being discussed on the ietf-behave@list.sipfoundry.org
mailing list
1. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [2].
2. Introduction
A major issue in working with NAT traversal solutions for various
protocols is that NATs behave in many different ways. This draft
describes the results of testing several residential style NATs.
3. Descriptions of Tests
3.1 UDP Mapping
This test sends STUN packets from the same port on three different
internal IP addresses to the same destination. The source port on
the outside of the NAT is observed. The test records whether the
port is preserved or not and whether all the mapping get different
ports.
A second set of tests checks out how the NAT maps ports above and
below 1024.
Tests are run with a group of several consecutive ports to see if the
NAT preserves port parity.
3.2 UDP Filtering
This test sends STUN packets from the same port on three different
internal IP addresses to the same destination. It then tests whether
places on the outside with 1) a different port but the same IP
address and then 2) a different port and a different IP address can
successfully send a packet back to the sender.
3.3 UDP Hairpin
This test sends a STUN packet from the inside to the outside to
create a mapping and discover the external source address called A.
It does the same thing from a different internal IP address to get a
second external mapping called B. It then sends a packet from A to B
and B to A and notes if these packets are successfully delivered from
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one internal IP address to the other.
3.4 ICMP
A device on the inside sends a packet to an external address that
causes an ICMP Destination Unreachable packet to be returned. The
test records whether this packet makes it back through the NAT
correctly.
3.5 Fragmentation
The MTU on the outside of the NAT is set to under 1000; on the inside
it is set to 1500 or over. Then a 1200 byte packet is sent to the
NAT. The test records whether the NAT correctly fragments this when
sending it. Another test is done with DF=1. An additional test is
done with DF=1 in which the adjacent MTU on the NAT is large enough
the NAT does not need to fragment the packet but further on, a link
has an MTU small enough that an ICMP packet gets generated. The test
records whether the NAT correctly forwards the ICMP packet.
In the next test a fragmented packet with the packets in order is
sent to the outside of the NAT, and the test records whether the
packets are dropped, reassembled and forwarded, or forwarded
individually. A similar test is done with the fragments out of
order.
3.6 UDP Refresh
A test is done that involves sending out a STUN packet and then
waiting a variable number of minutes before the server sends the
response. The client sends different requests with different times
on several different ports at the start of the test and then watches
the responses to find out how long the NAT keeps the binding alive.
A second test is done with a request that is delayed more than the
binding time but every minute an outbound packet is sent to keep the
binding alive. This test checks that outbound traffic will update
the timer.
A third test is done in which several requests are sent with the
delay less than the binding time and one request with the delay
greater. The early test responses will result in inbound traffic
that may or may not update the binding timer. This test detects
whether the packet with the time greater than the binding time will
traverse the NAT which provide the information about whether the
inbound packets have updated the binding timers.
An additional test is done to multiple different external IP
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addresses from the same source, to see if outbound traffic to one
destination updates the timers on each session in that mapping.
3.7 Multicast and IGMP
Multicast traffic is sent to the outside of the NAT, and the test
records whether the NAT forwards it to the inside. Next an IGMP
Membership Report is sent from inside. The test records whether the
NAT correctly forwards it to the outside and whether it allows
incoming multicast traffic.
3.8 Multicast Timers
The test records how long the NAT will forward multicast traffic
without receiving any IGMP Membership Reports and whether receiving
Reports refreshes this timer.
3.9 TCP Timers
TBD: Measure time before ACK, after ACK, and after FIN and RST.
3.10 TCP Port Mapping
Multiple SYN packets are sent from the same inside address to
different outside IP addresses, and the source port used on the
outside of the NAT is recorded.
3.11 SYN Filtering
Test that a SYN packet received on the outside interfaces that does
not match anything gets discarded with no reply being sent. Test
whether an outbound SYN packet will create a binding that allows an
incoming SYN packet.
4. Observations
Several NATs attempt to use the same external port number as the
internal host has used. This is referred to as port preservation.
Some of the NATs that do this were found to have different
characteristics depending on whether the port was already in use or
not. This was tested by running the STUN tests from a particular
port on one internal IP address and then running them again from the
same port on a different internal IP address. The results from the
first interface, where the port was preserved, are referred to as the
primary type; while the results from the second interface, which did
not manage to get the same external port because it was already in
use, are referred to as the secondary type. On most NATs the
secondary type is the same as the primary but on some it is
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different; these are referred to as nondeterministic NATs, since a
client with a single internal IP address cannot figure out what type
of NAT it is.
There are several NATs that would be detected as address restricted
by the STUN tests but are not. These NATs always use the same
external port as the internal port and store the IP address of the
most recent internal host to send a packet on that port. The NATs
then forward any traffic arriving at the external interface of the
NAT on this port to the internal host that has most recently used it.
These NATs are labeled "Bad" in the result table since they do not
meet the definitions of NAPT in RFC 3022. Interestingly, as long as
the clients behind the NAT choose random port numbers, they often do
work. STUN detects these NATs as address restricted although they
are really not address restricted NATs. This type of NAT is easily
detected by sending a STUN packet from the same port on two different
internal IP addresses and looking at the mapped port in the return.
If both packets have been mapped to the same external port, the NAT
is of the Bad type.
Another important aspect of a NAT for some applications is whether it
can send media from one internal host back to another host behind the
same NAT. This is referred to as supporting hairpin media.
It was rumored that some NATs existed that looked in arbitrary
packets for either the NATs' external IP address or the internal host
IP address - either in binary or dotted decimal form - and rewrote it
to something else. STUN could be extended to test for exactly this
type of behavior by echoing arbitrary client data and the mapped
address but sending the bits inverted so these evil NATs did not mess
with them. NATs that do this will break integrity detection on
payloads.
To help organize the NATs by what types of applications they can
support, the following groups are defined. The application of using
a SIP phone with a TLS connection for signaling and using STUN for
media ports is considered. It is assumed the RTP/RTCP media is on
random port pairs as recommended for RTP.
Group A: NATs that are deterministic, not symmetric, and support
hairpin media. These NATs would work with many phones behind
them.
Group B: NATs that are not symmetric on the primary mapping. This
group would work with many IP phones as long as the media ports
did not conflict. This is unlikely to happen often but will
occasionally. Because they may not support hairpin media, a call
from one phone behind a NAT to another phone behind the same NAT
may not work.
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Group D: NATs of the type Bad. These have the same limitations of
group B but when the ports conflict, media gets delivered to a
random phone behind the NAT.
Group F: These NATs are symmetric and phones will not work.
5. Results
The following table shows the results from several NATs. The NATs
tested include some random ones the author had lying around as well
as every NAT that could be purchased in February 2004 in the San Jose
Fry's, Best Buy, CompUSA, and Circuit City. Clearly this is not a
very good approximation to a random sample. It is clear that the
NATs widely purchased in the US are different from what are available
in Japan and Europe.
In the following table the Prim column indicates the primary type of
the NAT. A value of Port indicates port restricted, Cone is a full
cone, Bad is described in the next section, Symm is Symmetric, and
Addr is Address restricted. The Hair column value of Y or N
indicates whether the NAT will hairpin media. The Pres column
indicates whether the NAT attempts to preserve port numbers. The Sec
column indicates the secondary type of the NAT, and a value of Same
indicates it is the same as the primary type. The Grp indicates the
group that this NAT falls into.
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Vendor Model Firmware Prim Sec Hair Pres Grp
Airlink ASOHO4P V1.01.0095 Port Symm N Y B
Apple Air Base V5.2 Cone Same Y N A
Belkin F5D5321 V1.13 Port Same N N B
Cisco IOS Port Symm -
Cisco PIX Port Same -
Corega BAR Pro2 R1.00 Feb 21 2003 Cone -
DLink DI-604 2.0 Jun 2002 Cone Same N N B
DLink DI-704P 2.61 build 2 Cone Same Y N A
Dlink DI-804 .30, Tue,Jun 24 20 Cone Same Y N A
Hawkings FR24 6.26.02h Build 004 Bad Same Y Y D
Linksys BEFSR11 Port B
Linksys BEFSR11 V2 1.42.7, Apr 02 200 Port B
Linksys BEFSR41 v1.44.2 Port B
Linksys BEFSR81 2.42.7.1 June 2002 Addr Same N Y B
Linksys BEFSRU31 Port B
Linksys BEFSX41 1.44.3, Dec 24 200 Port B
Linksys BEFVP41 1.41.1, Sep 04 200 Port B
Linksys BEFW11S4 1.45.3, Jul 1 2003 Port B
Linksys WRT54G 1.42.2 Port Symm N Y B
Linksys WRT55AG 1.04, Jun.30, 2003 Port B
Linksys WRV54G 2.03 Port Same N Y B
Microsoft MN-700 02.00.07.0331 Cone Same N N B
Netgear FVS318 V1.4 Jul. 15 2003 Port Same N N B
Netgear RP114 3.26(CD.0) 8/17/20 Cone -
Netgear RP614 4.00 April 2002 Cone Same Y N A
NetworkEver NR041 Version 1.0 Rel 10 Symm Same N N F
NetworkEver NR041 Version 1.2 Rel 03 Bad Same Y Y D
SMC 2804WBRP-G v1.00 Oct 14 2003 Port Symm Y Y B
SMC 7004ABR V1.42.003 Port Same N N B
SMC 7004VBR v1.03 Jun 12, 2002 Cone -
Toshiba WRC-1000 1.07.03a-C024a Port Cone N Y B
umax ugate-3000 2.06h Port -
US Robotics USR8003 1.04 08 Cone Same N N B
ZOT BR1014 Unknown Bad Same N Y D
Since this testing was done, some additional testing and shopping
sprees in France and Taiwan have provided the following results.
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Vendor Model Firmware Prim Sec Hair Pres Grp
Netgear MR814v2 Version 5.01 Bad Same Y Y D
Cisco PIX 515 6.3(3) Port Same N N B
Dynex DX-E401 1.03 Cone Same Y N A
Asante FR1004 R1.13 V2 Cone Same N N B
Linksys BEFSR81 2.42.7.1 Addr Note 1 N Y B
Lanner BRL-04FPU Cone Same N N
AboCom CAS3047 Port Same N Y
Lemmel LM-IS6400B Port Same N Y
The NAT with a secondary type of "Note 1" is particularly weird. The
primary connection is address restricted. If a second host uses this
same port, it also gets an Address Restricted, but when a third host
uses this same port, it gets Symmetric.
Another good source of information for behavior of various NATs is
the NATCECK [9] and STUNT [8] web pages.
Open Issue: How should we arrange all the results? There are going to
be too many to put it as one row per device.
6. Discussion
It is clear from discussions with various vendors and watching how
tests have changed over the years that symmetric is becoming less
common. This change is being driven primarily by the desire to make
online gaming work; many games use methods similar to STUN for NAT
traversal. The only symmetric NAT found was an old device. More
recent versions of the software on the same device were not
symmetric. It is clear that other symmetric NATs are deployed, but
it is hard to find them.
7. Security Concerns
It is often assumed that symmetric NATs are more secure than port
restricted NATs. This is not true - they are identical from a
security point of view. They both only allow a packet to come inside
the NAT if the host inside has previously sent to the exact same
external IP and port. One can argue that cone is less secure than
port restricted, but this is not true if the attacker can spoof the
IP address, which is fairly easy to do in many cases. What level of
security can be expected from NATs at all is a strange and curious
topic. With all the NATs, if you allow packets out, packets can come
in, so don't be surprised if NATs provide less security than
anticipated.
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8. Open Issues
The hairpin media tests were done by having a single host use STUN to
find a public address on the NAT and then send media to itself and
see if it was received. It is possible that NATs might not hairpin
media to the same host but would hairpin media to another host behind
the same NAT. It is possible that because of this, the hairpin
results reported here might be wrong.
This sample set of NATs is very US-centric: D-Link, Linksys, and
Netgear dominate the US consumer market. It would be good to get
more results from other places.
These test results should be verified by another group. This has not
been done yet.
This draft should be moved to be consistent with the classification
in [11].
9. Acknowledgments
Many people and several mailing lists have contributed to the
material on understanding NATs in this document. Many thanks to
Larry Metzger, Dan Wing, and Rohan Mahy. The STUN server and client
is open source and available at http://sourceforge.net/projects/stun,
and thank you to Jason Fischl who runs the public STUN server at
larry.gloo.net. Thanks to Yutaka Takeda who tested and found bugs
and Christian Stredicke for getting people thinking. Thanks to
Francois Audet for catching mistakes, verifying several results, and
finding the very strange non-deterministic nature in the BEFSR81.
The work of the various people on STUN Client and Server [6], NATCECK
[10], and STUNT [7] has greatly helped this work.
10. References
10.1 Normative References
[1] Rosenberg, J., Weinberger, J., Huitema, C. and R. Mahy, "STUN -
Simple Traversal of User Datagram Protocol (UDP) Through Network
Address Translators (NATs)", RFC 3489, March 2003.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
10.2 Informative References
[3] Daigle, L. and IAB, "IAB Considerations for UNilateral
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Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[4] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.
[5] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
[6] Jennings, C., "STUN Client and Server:
http://www.vovida.org/applications/downloads/stun", February
2005.
[7] Guha, S. and P. Francis, "STUNT
http://nutss.gforge.cis.cornell.edu/stunt.php", February 2005.
[8] Guha, S. and P. Francis, "STUNT Results
http://www.guha.cc/saikat/stunt-results.php", February 2005.
[9] Ford, B. and D. Andersen, "Nat Check Results
http://bgp.lcs.mit.edu/~dga/view.cgi", February 2005.
[10] Ford, B. and D. Andersen, "Nat Check
http://midcom-p2p.sourceforge.net", February 2005.
[11] Audet, F. and C. Jennings, "NAT Behavioral Requirements for
Unicast UDP", draft-ietf-behave-nat-udp-00 (work in progress),
January 2005.
[12] Wing, D., "IGMP Proxy Behavior", draft-wing-behave-multicast-00
(work in progress), October 2004.
[13] Sivakumar, S., "NAT Behavioral Requirements for TCP",
draft-sivakumar-behave-nat-tcp-req-00 (work in progress),
January 2005.
Author's Address
Cullen Jennings
Cisco Systems
170 West Tasman Drive
Mailstop SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 421 9990
EMail: fluffy@cisco.com
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